Aviation and aerospace vehicles are often first coated with a primer based on epoxy resin or polyurethane and a curing agent, and overcoated with a curable two-component polyurethane topcoat that has resistance to water and organic solvents such as aviation fuel and/or hydraulic fluids. Primer is often applied to metallic surfaces for enhanced corrosion resistance or to polymeric surfaces to enhance the adhesion of subsequently applied layers, while topcoats are often chosen to achieve properties such as solvent resistance, weatherability, abrasion resistance, hardness, and/or aesthetics.
During use, the topcoat surface can age. The term “aged surface” as used herein refers to a surface whose physical and/or chemical properties are degraded compared to those of the new surface. Aging of surfaces can result from the effects of, for example, mechanical abrasion, impact, water, solvents, temperature, radiation, moisture, weathering and/or other mechanisms that cause degradation of the physical and/or chemical properties and/or the aesthetic qualities of a surface. “Aged surface” as used herein also includes surfaces that are degraded from their initially applied condition, wherein the degradation results from a relatively brief exposure to a stress, as compared to aging, which typically results from long term exposure to one or more stress conditions. For example, a surface can become damaged by mechanical stress such as abrasion, impact, and the like, thermal stress, such as exposure to excessive temperatures, exposure to solvents and/or chemical stress such as exposure to irradiation, and the like. In practice, it can be useful to refinish an aged surface to restore the physical, chemical, and/or aesthetic qualities of the surface.
Adhesion between the new coating or the “refinish” or “refinished” coating and the aged surface can contribute to the overall performance of the coating. When a part is first coated, the quality of the coating can exhibit the same or similar physical and chemical properties across the surface. However, for example, on used, aged, and/or damaged surfaces, and particularly for large surfaces such as an aircraft, a surface to be coated and/or refinished may not be homogeneous.
When a large surface such as that of an aircraft is to be refinished, the topcoat can be removed using a stripper before repainting. To avoid having to pre-treat a fresh metallic surface and reapply a new primer coating, it can be useful to only remove the topcoat layer before repainting. Swellable, strippable topcoats and processes for removing a topcoat while leaving a cured primer coating on a metallic surface are disclosed in U.S. Pat. No. 6,217,945, incorporated herein by reference. The stripping process can use solvents and generate chemicals waste, however, which may be undesirable.
In certain applications, it can be useful to apply a polyurethane topcoat to an aged polyurethane surface without first removing the aged coating. However, adhesion of polyurethane coatings to aged coatings may be poor. Surface phenomena of the aged coating, such as chalking, the presence of surface micropores, and/or entrapped water can contribute to the poor adhesion of new polyurethane topcoats to an aged coating.
To facilitate and/or enhance the adhesion of a new polyurethane topcoat to an aged polyurethane surface, the aged surface can be mechanically abraded prior to applying the new polyurethane topcoat. Mechanical abrasion of an aged surface can be accomplished by, for example, sanding. Sanding large surfaces, such as those of an aircraft, can be costly, time consuming, and/or difficult to control to the extent necessary to facilitate uniform adhesion of a newly applied topcoat. Furthermore, the process of mechanical abrasion can release particulates comprising potentially harmful and/or toxic chemicals.
Alternative methods to solvent stripping or mechanical sanding are therefore desired, particularly for aviation and aerospace applications.